DESCRIPTION: Toxoplasmic encephalitis (TE) is one of the most common life threatening central nervous system infections in HIV infected patients with advanced disease. Although incidence of TE as a result of cART (combination antiretroviral therapy) has decreased substantially, it continues to be a problem in some countries with high prevalence of infection. TE is still the most common cerebral complication in AIDS patients. It is believed that even in post ART era, fatal toxoplasmosis remains a significant problem in HIV infected individuals. Studies conducted during last two decades, including those carried out in our laboratory have demonstrated a critical role for CD8+ T cells, both in protective immunity generated against vaccine strains and control of infection in the mice carrying chronic Toxoplasma infection. However, despite induction of strong CD8+ T cell immunity, susceptible strains of animals are unable to prevent reactivation of latent infection and they develop TE. Recent studies from our laboratory have demonstrated that CD8+ T cells from the susceptible strain of mice become exhausted and lose their functional ability to keep chronic infection under control. CD8+ T cell exhaustion was attributed to graded up-regulation of PD-1 (a well-known inhibitory molecule) expression on these cells. Although blockade of PD-1 interaction with its ligand PDL-1 invigorated CD8+ T cell response, highly exhausted cells could not be rescued. Preliminary data for the proposal demonstrates that in addition to PD-1, CD8+ T cells from susceptible animals exhibited increased expression of other inhibitory molecules like LAG-3, 2B4 and CTLA-4. Thus blockade of multiple inhibitors may be needed to restore CD8+ T cell functionality. Moreover, mechanism responsible for upregulation of inhibitory receptors leading to CD8+ T cell exhaustion needs to be evaluated. The proposal has three specific aims. In specific aim 1, kinetics and pattern of multiple inhibitory receptors expressed by CD8+ T cells from infected animals will be performed. This will provide important information about the antibody cocktail needed for reversing the exhaustion so that reactivation of latent infection is prevented. In specific aim 2, underlying mechanism responsible for CD8+ T cell exhaustion in mice carrying chronic Toxoplasma infection will be evaluated. Preliminary data suggests that optimal IL-21 levels play critical role in maintaining functional CD8+ T cell response. In this specific aim important role of IL-21 producing CD4+ T cells in programming of CD8+ T cells for long-term functionality will be determined. Finally in the third specific aim, role of CD40 agonist treatment as a supplemental therapy to antibody blockade of inhibitory molecules will be assayed. Information generated from these studies will be highly beneficial to develop therapeutic regimen for preventing TE which as stated above continues to be a serious problem for HIV infected population.